128 Therapeutic Uses of Ionizing Radiation

How Radiation Therapy Works

Radiation therapy (also called radiotherapy) is the use of high-energy ionizing radiation to destroy cancer cells or prevent them from growing and dividing. It is one of the most common treatments for cancer and is used in approximately half of all cancer patients at some point during their care. Depending on the type and stage of the disease, radiation therapy may be used alone or combined with surgery, chemotherapy, immunotherapy, or targeted drug therapies.

The goal of radiation therapy is to deliver a high radiation dose to a tumor while minimizing exposure to the surrounding healthy tissues. Advances in medical imaging, computer treatment planning, and radiation delivery systems now allow physicians to target tumors with remarkable precision.

Why Radiation Damages Cancer Cells

Ionizing radiation deposits energy within cells, producing ions and highly reactive free radicals that damage DNA. Although both healthy and cancerous cells are affected, cancer cells are generally less able to repair this damage. As a result, they are more likely to stop dividing or undergo programmed cell death (apoptosis).

Healthy tissues are usually better able to repair radiation-induced damage between treatments. This difference in repair capability forms the biological basis of radiation therapy.

The Therapeutic Ratio

An important goal of radiation therapy is to maximize the destruction of cancer cells while minimizing injury to healthy tissue. This balance is described by the therapeutic ratio, which compares the biological damage produced in the tumor with the damage produced in normal tissues.

Modern radiation oncology seeks to improve the therapeutic ratio through careful treatment planning, accurate patient positioning, advanced imaging, and highly focused radiation beams.

By directing multiple beams toward the same target, each individual beam deposits only a modest dose as it passes through healthy tissue. At the point where all of the beams intersect, however, the doses add together to produce a much larger dose within the tumor. This geometric approach allows physicians to concentrate radiation where it is needed most while reducing exposure to nearby organs.

External Beam Radiation Therapy

The majority of patients receiving radiation therapy are treated with external beam radiation therapy (EBRT), in which radiation is produced outside the body and directed toward the tumor. Modern treatment systems use a linear accelerator (LINAC), a machine that accelerates electrons to high energies. These electrons either strike a metal target to produce high-energy x-rays or are used directly for treating tumors near the surface of the body.

Before treatment begins, patients undergo detailed imaging studies, usually including computed tomography (CT) and sometimes magnetic resonance imaging (MRI) or positron emission tomography (PET). These images allow physicians and medical physicists to identify the exact size, shape, and location of the tumor while also mapping nearby healthy organs that should receive as little radiation as possible.

Treatment Planning

Once the tumor has been identified, specialized treatment-planning software calculates how radiation beams should be directed to deliver the prescribed dose. The computer considers factors such as beam energy, beam angle, and the shape of the radiation field so that the highest dose is delivered to the tumor while minimizing exposure to surrounding tissues.

Most treatments use multiple beams entering the body from different directions. Although each individual beam deposits some radiation in healthy tissue, all of the beams intersect at the tumor, where their doses combine to produce the highest radiation dose.

Modern Radiation Delivery

Today's radiation therapy systems continuously rotate around the patient while adjusting the shape and intensity of the radiation beam. This allows the treatment to conform closely to the three-dimensional shape of the tumor.

Two commonly used techniques are:

• Intensity-Modulated Radiation Therapy (IMRT): The intensity of different parts of the radiation beam is varied to better match the shape of the tumor and spare nearby healthy organs.
• Volumetric Modulated Arc Therapy (VMAT): The linear accelerator rotates around the patient while continuously changing the beam shape, beam intensity, and dose rate, allowing highly conformal treatments to be delivered in just a few minutes.

Many treatment systems also perform imaging immediately before each treatment session. This process, known as image-guided radiation therapy (IGRT), verifies that the patient and tumor are correctly positioned before radiation is delivered. Even small positioning errors of a few millimeters can significantly affect treatment accuracy, making daily image guidance an important part of modern radiation therapy.

Brachytherapy and Targeted Radionuclide Therapy

Although external beam radiation therapy is the most common form of radiotherapy, some cancers can be treated more effectively by placing the radioactive source inside or very close to the tumor. This approach reduces the radiation dose delivered to healthy tissues while maintaining a high dose within the cancer.

Brachytherapy

Brachytherapy is a form of radiation therapy in which small radioactive sources are placed directly inside or next to a tumor. Because radiation intensity decreases rapidly with distance, tissues only a few centimeters away receive much smaller doses than the tumor itself.

The radioactive sources, often called seeds, are typically about the size of a grain of rice. Depending on the type of cancer, they may remain in the body permanently or be removed after treatment.

The primary advantage of brachytherapy is that the radiation source is located within or immediately adjacent to the tumor. As a result, the radiation dose falls off rapidly with distance, minimizing exposure to nearby healthy organs.

Targeted Radionuclide Therapy

Another approach uses radioactive substances that travel through the bloodstream and accumulate preferentially in certain tissues or tumors. These treatments combine the principles of nuclear medicine and radiation therapy by delivering radiation from within the body.

One of the oldest and most successful examples is the treatment of thyroid cancer with radioactive iodine (¹³¹I). Because thyroid cells naturally absorb iodine to produce thyroid hormones, radioactive iodine is concentrated within both normal and cancerous thyroid tissue, where it destroys the cells from the inside.

Today, many targeted radionuclide therapies have been developed. Some radioactive isotopes are attached to antibodies or other molecules that specifically recognize proteins found on cancer cells. After injection into the bloodstream, these molecules carry the radioactive isotope directly to the tumor, reducing radiation exposure to most healthy tissues.

Because these treatments rely on the biological behavior of cancer cells rather than simply directing radiation from outside the body, they represent an important step toward personalized cancer therapy. Researchers continue to develop new radioactive drugs capable of delivering radiation even more selectively to tumors while reducing side effects.

Emerging Technologies in Radiation Therapy

Radiation therapy has advanced tremendously over the past several decades. Improvements in medical imaging, computing power, and accelerator technology now allow physicians to deliver radiation with greater precision than ever before. Researchers continue to develop new techniques that maximize damage to tumors while further reducing side effects for patients.

Proton Therapy

Most external beam radiation therapy uses high-energy x-rays, which deposit energy as they enter the body, pass through the tumor, and continue beyond it. In contrast, proton therapy uses beams of positively charged protons. Unlike x-rays, protons deposit relatively little energy until they reach a specific depth, where they release most of their energy in a phenomenon known as the Bragg peak.

By adjusting the energy of the proton beam, physicians can position the Bragg peak within the tumor, reducing the radiation dose delivered to healthy tissues beyond the treatment area.

Heavy-Ion Therapy

Some specialized treatment centers use protons, neutrons, or heavier charged particles such as carbon ions. Like protons, these particles exhibit a Bragg peak that allows highly localized dose delivery. In addition, they produce denser ionization along their tracks, resulting in greater biological effectiveness against certain tumors that are resistant to conventional x-ray therapy.

Because heavy-ion facilities require very large particle accelerators and specialized infrastructure, they remain less common than conventional radiation therapy or proton therapy.

FLASH Radiotherapy

One of the newest areas of radiation therapy research is FLASH radiotherapy. In this experimental technique, the entire treatment dose is delivered in an extremely short burst lasting only fractions of a second.

Early laboratory and clinical studies suggest that FLASH radiotherapy may reduce damage to healthy tissues while maintaining the ability to destroy cancer cells. Researchers are actively investigating why this effect occurs and whether it can be applied safely to a wider range of cancers.

Radiation Therapy in Perspective

Radiation therapy demonstrates how the principles of nuclear physics and electromagnetism can be applied to improve human health. By understanding how ionizing radiation interacts with matter, healthcare professionals can safely deliver treatments that destroy cancer cells while protecting healthy tissues. Modern radiation therapy continues to evolve, offering more precise, more effective, and more personalized treatments than ever before.

Healthcare Connection: Neutrons in Cancer Therapy and Medical Imaging

Although x-rays and proton beams are used most commonly in radiation oncology, neutrons also have important medical applications. One promising technique is Boron Neutron Capture Therapy (BNCT), which combines a boron-containing drug with a beam of low-energy neutrons.

Before treatment, a compound containing the stable isotope ¹⁰B is administered to the patient. Ideally, the compound accumulates preferentially within cancer cells. When these boron atoms capture neutrons, they undergo a nuclear reaction that produces energetic alpha particles and lithium nuclei:

[latex]^{10}\mathrm{B}+n \rightarrow ^7\mathrm{Li}+\alpha+\gamma[/latex]

The alpha particle and lithium nucleus travel only a few micrometers—approximately the diameter of a single cell—depositing nearly all of their energy within the cancer cell that captured the neutron. This highly localized energy deposition can destroy tumor cells while limiting damage to nearby healthy tissue.

Neutrons are also valuable in medical imaging and biomedical research. Because neutrons interact primarily with atomic nuclei rather than electrons, they provide information that complements conventional x-ray imaging. In particular, neutrons are highly sensitive to hydrogen-rich materials such as water and soft tissues, making neutron imaging useful for studying biological specimens, implants, and the movement of fluids within medical devices.

Although neutron-based therapies and imaging remain available at only a limited number of specialized centers worldwide, ongoing advances in accelerator technology and detector systems are expanding their potential role in medicine.

Section Summary

• • Radiation therapy uses ionizing radiation to damage the DNA of cancer cells, preventing them from dividing and leading to cell death.
  • Cancer treatment is based on the therapeutic ratio, which aims to maximize damage to the tumor while minimizing radiation exposure to healthy tissues.
  • Most radiation therapy is delivered using external beam radiation therapy (EBRT), in which a linear accelerator (LINAC) generates high-energy x-rays or electrons that are carefully shaped and directed toward the tumor.
  • Modern treatment planning combines CT, MRI, and PET imaging with computer modeling to optimize radiation delivery. Techniques such as intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and image-guided radiation therapy (IGRT) improve treatment precision while reducing dose to nearby healthy tissues.
  • Fractionation, in which the total radiation dose is delivered over many treatment sessions, allows normal tissues time to repair while reducing the ability of cancer cells to recover.
  • Brachytherapy places radioactive sources directly within or near a tumor, producing high local doses with rapid dose falloff outside the treatment area.
  • Targeted radionuclide therapy uses radioactive isotopes attached to molecules that selectively accumulate in cancer cells, delivering radiation from within the body.
  • Emerging techniques such as proton therapy, heavy-ion therapy, and Boron Neutron Capture Therapy (BNCT) further improve the ability to concentrate radiation within tumors while sparing surrounding healthy tissues.
  • Advances in radiation therapy continue to improve cancer treatment through innovations in medical physics, imaging, engineering, and radiopharmaceutical development.